Conventionally, in this kind of chip resistor, the upper surface of the protective coat is not flat but projects largely at the center portion. Therefore, in moving the chip resistor using a collet of a vacuum suction type, the collet sometimes does not adhere firmly to the protective coat or the protective coat sometimes cracks.
In addition to the above, the conventional structure has the following drawback. Each of the external connection terminals of the conventional chip resistor includes a portion extending on the upper surface of the insulating substrate (hereinafter this portion is referred to as “upper electrode”). The upper electrode is held in contact with the resistor film. The upper electrode is made of conductive paste mainly composed of silver, and the thickness is made relatively small to facilitate the formation of the resistor film. With this structure, however, the upper electrode may be corroded by air, and in a serious case, the upper electrode is broken. This is because silver, which is the main component of the upper electrode, reacts with sulfur gas (such as hydrogen sulfide) in the atmosphere to become silver sulfide.
Patent Documents 1 and 2 described below propose techniques for coping with the above-described drawbacks. According to these documents, a relatively thick auxiliary upper electrode is formed on each of the upper electrodes (hereinafter referred to as “main upper electrode”) connected to the resistor film. With this structure, the stepped portion between the center and the opposite ends of the upper surface of the substrate can be eliminated or reduced. Further, since the main upper electrode is covered by the auxiliary upper electrode, corrosion of the main upper electrode is expected to be reduced.
Patent Document 1: JP-A-H08-236302
Patent Document 2: JP-A-2002-184602
However, it has been found that, even with the above-described conventional structures, it is difficult to reliably prevent corrosion of the main upper electrode. Specifically, according to Patent Document 1, the auxiliary upper electrode is made of silver-based conductive paste. With this structure, corrosion due to e.g. sulfur components in the atmosphere may occur at the boundary between the auxiliary upper electrode and the protective coat, and the corrosion progresses to the main upper electrode positioned below.
According to Patent Document 2, the auxiliary upper electrode is made of nickel-based conductive paste. With this structure, damages such as cracking may occur at the boundary between the auxiliary upper electrode and the protective coat. Through the damaged portion, sulfur components in the atmosphere reach the main upper electrode to corrode the main upper electrode.